I. The Mendelian Principle of Purity of Germ-Cells and the Laws of Heredity Based on Ancestry.
Professor Weldon’s article is entitled “Mendel’s Laws of Alternative Inheritance in Peas.” This title expresses the scope of Mendel’s work and discovery none too precisely and even exposes him to distinct misconception.
To begin with, it says both too little and too much. Mendel did certainly determine Laws of Inheritance in peas—not precisely the laws Professor Weldon has been at the pains of drafting, but of that anon. Having done so, he knew what his discovery was worth. He saw, and rightly, that he had found a principle which must govern a wide area of phenomena. He entitles his paper therefore “Versuche über Pflanzen-Hybriden,” or, Experiments in Plant-Hybridisation.
Nor did Mendel start at first with any particular intention respecting Peas. He tells us himself that he wanted to find the laws of inheritance in hybrids, which he suspected were definite, and that after casting about for a suitable subject, he found one in peas, for the reasons he sets out.
In another respect the question of title is much more important. By the introduction of the word “Alternative” the suggestion is made that the Mendelian principle applies peculiarly to cases of “alternative” inheritance. Mendel himself makes no such limitation in his earlier paper, though perhaps by rather remote implication in the second, to which the reader should have been referred. On the contrary, he wisely abstains from prejudicial consideration of unexplored phenomena.
To understand the significance of the word “alternative” as introduced by Professor Weldon we must go back a little in the history of these studies. In the year 1897 Galton formally announced the Law of Ancestral Heredity referred to in the Introduction, having previously “stated it briefly and with hesitation” in Natural Inheritance, p. 134. In 1898 Professor Pearson published his modification and generalisation of Galton’s Law, introducing a correction of admitted theoretical importance, though it is not in question that the principle thus restated is fundamentally not very different from Galton’s[54]. It is an essential part of the Galton-Pearson Law of Ancestral Heredity that in calculating the probable structure of each descendant the structure of each several ancestor must be brought to account.
Professor Weldon now tells us that these two papers of Galton and of Professor Pearson have “given us an expression for the effects of blended inheritance which seems likely to prove generally applicable, though the constants of the equations which express the relation between divergence from the mean in one generation, and that in another, may require modification in special cases. Our knowledge of particulate or mosaic inheritance, and of alternative inheritance, is however still rudimentary, and there is so much contradiction between the results obtained by different observers, that the evidence available is difficult to appreciate.”
But Galton stated (p. 401) in 1897 that his statistical law of heredity “appears to be universally applicable to bi-sexual descent.” Pearson in re-formulating the principle in 1898 made no reservation in regard to “alternative” inheritance. On the contrary he writes (p. 393) that “if Mr Galton’s law can be firmly established, it is a complete solution, at any rate to a first approximation, of the whole problem of heredity,” and again (p. 412) that “it is highly probable that it [this law] is the simple descriptive statement which brings into a single focus all the complex lines of hereditary influence. If Darwinian evolution be natural selection combined with heredity, then the single statement which embraces the whole field of heredity must prove almost as epoch-making as the law of gravitation to the astronomer[55].”
As I read there comes into my mind that other fine passage where Professor Pearson warns us
“There is an insatiable desire in the human breast to resume in some short formula, some brief statement, the facts of human experience. It leads the savage to ‘account’ for all natural phenomena by deifying the wind and the stream and the tree. It leads civilized man, on the other hand, to express his emotional experience in works of art, and his physical and mental experience in the formulae or so-called laws of science[56].”